Having been awarded the first 13km long reach of the 4.9m i.d. segmentally lined underground mineral transport system (MTS) for the Woodsmith polyhalite potash mine in the UK, from Wilton at the materials handling facility near the shipping harbour on the northeast coast of the UK, to the intermediate working shaft at Lockwood Beck, the design-build contract to the Strabag-led group was subsequently extended to include the second and third TBM drives to complete the full 37km long MTS system from Wilton to the mine site via the intermediate access shaft at Lockwood Beck (Figs 1 and 2). The Strabag team will also engineer, procure and fit-out the mineral transport system for operation.

Following a period in design and mobilisation, Strabag started excavation for MTS in early 2019 with launch of the first of two 6m o.d. TBMs in June 2019. The polyhalite resource is located 1,500m beneath the North York Moors National Park and the mineral once extracted will be lifted to an underground cavern to transport to the Teesside processing plant and export shipping harbour via the 37km long 4.9m i.d. x 37km long MTS (Fig 3 and 4). The first TBM, a machine supplied by Herrenknecht, is proceeding on a 3.5 to 0.9% down gradient slope from the portal in Wilton to the intermediate shaft cavern at Lockwood Beck. The same TBM is planned to progress from the Lockwood Beck shaft towards the mine head with a second TBM, on a third drive, progressing from the mine head towards an in-tunnel junction to complete the remaining reach of the MTS tunnel. The second TBM is due to be procured and launched in 2022.

In a presentation to the virtual Salzburg Geomechanics Colloquium in October 2020, Ernst Gschnitzer, Julian Wenger and Andy Raine of Strabag AG, UK Branch, explained that excavation of the MTS started at Wilton with a 300m long open cut area into a 100m long shotcrete lined SCL heading. Originally the SCL heading was to be 800m long through the upper mixed face geological deposits, but the distance could be shortened to 100m due to slightly better than anticipated rock conditions. The SCL tunnel was completed in April 2019 and in June 2019 the walked in TBM reached the rock face to begin full-face excavation in favourable Redcar Mudstone. By early October 2020, 10km of this first 13km long drive was complete.

Fig 4. Cross sections of the TBM excavation operation (left) and Fig 5. of the permanent mineral transport conveyor system (right)

Geological data, geophysical borehole surveys, seismic traverses, surface mapping and 12 borehole samples along the MTS route were studied to confirm the alignment. Data from historical and recent mining activities were also available and studied. During excavation, fault sensors on the TBM, shape arrays and triaxial nodes to detect movements, probe drilling in certain areas, in-situ stress measurements and face scanning data are adding to the understanding and control of the geological conditions.

The 6m diameter single-shield Herrenknecht rock TBM has an installed power of 2,900kW, a cutterhead power of 2,100kW, delivered via six 350kW motors, and a maximum cutterhead rotation of up to 9 rev/min. Cutterhead torque is 3,281kNm at 5.81 rev/min and 4,758kNm maximum at breakout. Thrust is between nominal 62,068kN and 73,890kN.

The 6m o.d. tunnel is lined to 4.9m i.d. with five 350mm thick segments and a universal large key in each 1.5m long ring. A 200mm annular gap with overcut allows for any potential squeezing and rock blocking interaction with the shield beneath the 370m deep cover. The lining is of three types with strongest casting of segments, reinforced with 125kg/m³ steel rebar cages and a concrete grade C50/60, is designed to take the overburden pressure and up to 29 bar of hydrostatic pressure.

First 6m single-shield rock TBM procured for the MST drives

Within the single service rail track heading, logistics rely on movable and fixed California crossings. After completion of excavation, the track system will become the permanent infrastructure, requiring only small modifications and maintenance. The permanent fit-out includes the MTS mineral haulage conveyor system which is divided into two reaches: the 24km reach from the mine head to the Lockwood Beck cavern; and a 13km long conveyor from Lockwood Beck to the Teesside processing plant.

After launch in June 2019, and due to the strict selection process of the TBM, advance rates to date have been high with a consistent daily advance rate, with a best excavation day of 52.65m in 24h and the best month of 1,012m in October 2020. With this high performance, the project was 145 days and 2.5km ahead of the programme in November 2020. Nevertheless, some downtime periods have been experienced to manage probe drillings in fault zones, extend cable and the continuous conveyor muck haulage system and clear clogging of the cutterhead.

Development of the mineral transport tunnel as a vital part of the new potash mine in North Yorkshire, UK, has moved into construction with award of a design-build contract for the first TBM drive of the system to Strabag.

Fig 1. First of three TBM drives of the 37km long MTS tunnel awarded to Strabag

Awarded by mine owner Sirius Minerals, the first drive is the 13km long reach of the 4.7m segmentally lined tunnel from Wilton, at the materials handling facility near the shipping harbour on the northeast coast of the UK, to the intermediate working shaft at Lockwood Beck. A second and third TBM drive, to be awarded under separate contracts, will complete the material transport system (MTS) tunnel from Lockwood Beck towards the Woodsmith mine site and from the mine site towards Lockwood Beck (Fig 1).

The 4.7m diameter x 37km long MTS tunnel will house a continuous conveyor to deliver the high quality polyhalite potash from the deep level mining operation near Whitby to the processing plant near Redcar on Teeside (Fig 2). The underground conveyor system is a low environmental impact alternative to delivering the product by trucks across the countryside and through the North York Moors National Park in which the new minehead is located. The MTS was a major concession for securing planning permission to develop the mine.

The Strabag proposal for its MTS tunnel contract is reported as providing a compelling offer in terms of price, schedule, safety management and risk allocation and that the contract awarded is a lump sum with fixes rates for tunnel advance. A statement by Sirius Minerals adds that the price is based on a defined and agreed geotechnical baseline report (GBR) with firm pricing for a range of expected support classes within the build and that the cost of the work will be incurred in British GBP currency.

Fig 2. Woodsmith Mine will convey potash raw material to the processing plant at Redcar via continuous conveyor in the 37km long transport tunnel

In announcing the contract award, Sirius Minerals confirmed that site mobilisation is to begin immediately and add to work already started on other mine development contracts.

At the main mine head site, excavation is advancing of the 1,500m deep x 6.75m i.d. production shaft to the 70m thick seam of polyhalite, and the 1,500m deep x 6.75m i.d. working shaft to which the MTS tunnel will connect for onward transport of the extracted polyhalite to the planned processing plant and port facility at Redcar.

At the same time, Fugro is continuing geotechnical and geophysical investigations at the mine head and at sites along the route of the MTS tunnel. These add to a series of geotechnical investigations across the project between 2013 and 2016 by the company which included preliminary geotechnical and hydrological investigations at the mine site, seven deep boreholes along the transport corridor and at the export dock, and a challenging deep inclined hole for fault characterisation.

Works advancing at the mine head site as of February 2018

Award of the first TBM drive of the MTS to Strabag cancels out selection by Sirius in mid-2016 of the Hochtief/Murphy JV as the preferred bidder for the 37km long transport tunnel. Strabag was part of the competitive tendering processes for a single MTS development contract in those early stages of the project along with other rivals for the lucrative contract. Under the current round of evaluations, bidders were invited to develop technical solutions and bids for the limited first drive of the MTS and as part of the Phase 1 development funding and financial investment.

In accouncing the preferred bidder status, the 50/50 Hochtief/Murphy JV said it was "to carry out front-end engineering designs over a 12 month period" and enter into contract negotiations for the final design and build contract. "Following award of the delivery contract," it continued, "the joint venture will commence the construction of the mineral transport system."

Sirius is said to be continuing to work closely with a number of parties in relation to the second and third drives of the MTS with a view to having those scopes of work awarded in conjunction with the completion of Phase 2 financing later this year (2018). The mining company is aiming to achieve first product from the mine by the end of 2021 and ramp up to an initial production capacity of 10 million tonnes per annum (Mtpa) by 2024.

Current schedule for development of the new mine and its facilities

Sirius Minerals Plc is a privately owned, publically traded company on the London Stock Exchange. After successful raising US$1.2 billion to finance Phase I construction, Phase II fundraising to raise up to a further US$3 billion is expected to come in the form of debt rather than issue of new equity with a syndicate of six banks - J P Morgan, Lloyds, EDC, Societe Generale, ING and Royal Bank of Scotland – raising a commercial tranche of up to US$1 billion and up to US$2 billion in UK Government guaranteed bonds. Commitment for Phase II funding is targeted for late 2018 for a first drawdown in 2019.

Stage 1 funding of US$1.2 billion was completed in 2016 and comprises £370 million of equity placing, US$400 million in convertible notes and a strategic investment commitment by Hancock Prospecting of Australia of US$300 with a US$250 million revenue royalty paying 5% for up to 13 Mtpa and 1% thereafter, and a US$50 million equity of 200 million shares to be determined by the placement price.